The large-scale production of oxygen, nitrogen and argon is typically carried out through cryogenic rectification. It has been known for some time that the use of structured packing inside the rectification columns offers distinct advantages over trays, which were the previous column internal of choice. The primary advantage of structured packings is their low pressure drop per theoretical stage of separation. This reduces the operating pressure of the columns, which in turn drops the pressure to which air must be compressed and thus the power requirements for the plant. The use of packing also enables the use of extended argon columns for the production of essentially oxygen-free argon directly from the plant.
Structured packings do have disadvantages over trays. Packed columns are typically more expensive to manufacture than their trayed counterparts. In addition, while the height of packing per theoretical stage may be similar to trayed columns, the additional height required for collection and distribution of liquid means that industrial columns containing commercially available structured packings are taller than trayed columns. Increasing the height of the distillation columns has several drawbacks particularly for air separation. For example, heat leak into the cryogenic system increases as the column package height increases which must be overcome through the generation of additional refrigeration at the expense of power. In addition, liquid transfer from the higher pressure column to the lower pressure column may require liquid pumping or vapor lift if the height of the lower pressure column package is too high, adding complexity and inefficiency to the process. Vapor lift is the term used to describe the combination of a lower density stream, which is normally vapor, with a higher density stream, which is normally liquid, so as to reduce the mean fluid density and thus reduce the hydrostatic head contribution to pressure drop when transporting the stream to a higher elevation.
For these reasons cryogenic air separation plants employ very low tray spacing and low height equivalent of a theoretical plate (HETP) packing when compared to other typical industrial separations in the chemical industry. HETP is a term often used in the characterization of packed distillation columns and is used herein in the general sense that a lower HETP indicates that more mass transfer occurs in a given height of packing.
Crude argon, having an argon concentration of about 98 mole percent(%) or less, is produced by the cryogenic rectification of air. Argon comprises less than 1% of air. Typically air is separated into oxygen and nitrogen by use of a double column system comprising a high pressure column in heat exchange relation with a low pressure column. Argon has a boiling point intermediate to that of nitrogen and oxygen but closer to oxygen and will tend to concentrate in the lower regions of the low pressure column. At or near the point in the low pressure column that the concentration of argon is a maximum, a stream is withdrawn and passed into an argon column for rectification into a crude argon product. The argon concentration in the feed stream is typically 7-15% so that effective argon recovery can be attained by the argon column system. The remainder of the argon column feed stream comprises oxygen and nitrogen.
In the argon column the feed is separated by cryogenic rectification. The less volatile component, oxygen, is stripped from the rising vapor and argon concentrates at the top of the column. Any nitrogen present in the feed to the argon column will also concentrate at the top of the column since nitrogen is more volatile than both argon and oxygen. The crude argon product that leaves the top of the column generally comprises 95-98% argon. The remainder is essentially oxygen and nitrogen. The crude product is sent for further processing to produce high purity or refined argon. Oxygen is removed from the crude argon stream by mixing it with hydrogen and passing the mixture through a catalytic hydrogenation unit where the hydrogen and oxygen react to form water. The stream is then passed through a dryer to remove the water. Alternative methods for the removal of oxygen exist. Once the oxygen has been removed, nitrogen is separated from the argon stream by cryogenic distillation. The resulting high purity or refined argon having an oxygen concentration generally less than 2 ppm and a nitrogen concentration generally less than 2 ppm is now suitable for commercial use.
The capital and operating costs of producing refined argon from the argon column system are considerable. However, it is possible to produce essentially oxygen-free argon directly if sufficient equilibrium stages are provided in the argon column. Typically, the number of equilibrium stages that are provided in an argon column for the production of crude product is 40-50. This number must increase to 150 or higher to meet the oxygen concentration specification by distillation alone.
Production of nearly oxygen-free argon through extension of the argon column has some obvious advantages. Primarily, it does not require additional unit operations downstream, with the associated hardware and control requirements. However, the large number of equilibrium stages involved means that very tall columns are required. Currently, if nearly oxygen-free argon is sought, an additional column is placed in series with the crude column that would be present in a conventional argon producing plant. This second column, referred to as the superstaged column, must generate 100 or so equilibrium stages of separation, the crude column generating about 50. Vapor is taken from the top of the crude column and drawn into the base of the superstaged column. Liquid from the base of the superstaged column is pumped to the top of the crude column. Splitting the column has obvious economic drawbacks and clearly substantial savings could accrue from using a single column to perform the same task. However, the reality is that, with conventional structured packings, the additional expense of splitting the column is justified in view of the costs associated with building and erecting the taller column required if the crude column is to be eliminated.
Accordingly it is an object of this invention to provide a structured packing brick which may be employed in a column so that in operation the column may be used to carry out a given separation with a column height which is less than that which would be required for that separation using conventional structured packing as the column internals.